Republicans in Congress are blocking the Navy’s march toward a future in which jets and ships run on biofuels, saying the “green” most in question is whether the United States has the money, with a shrinking military budget, to invest in a pricey fuel experiment.

The Navy will demonstrate its “great green fleet” concept next month, a toe in the water toward easing the country away from dependence on unfriendly foreign oil. But opposition in Congress could push the effort onto the rocks after that.

Committees in both houses of Congress last month put language in the 2013 defense bill that would block the Pentagon from buying biofuels if they cost more than traditional petroleum.

The vote split mostly along party lines, with Republicans, including San Diego County Rep. Duncan Hunter, saying that proposed massive cuts to defense spending leave no room for anything but essentials.

“We’ll need to learn to do much more with much less. Experimenting with biofuels at such a significant cost to taxpayers will not put the military on better footing or increase effectiveness, next to other investments that are much higher on the priority list,” said Joe Kasper, Hunter’s spokesman.

The Navy, which sees its green initiative as a strategic move away from reliance on expensive Persian Gulf oil, spent $13.7 million on fuel made from used cooking oil and algae, blended with petroleum, for its upcoming green fleet voyage on July 18 and 19.

It was the biggest alternative fuel purchase ever for the U.S. government — 200,000 gallons of jet fuel and 700,000 gallons of ship diesel.

But it might also be the most expensive, costing nearly four times as much as traditional petroleum.

The cost of the biofuels calculated out at $15.25 a gallon in the November 2011 purchase, said Thomas Hicks, the Navy’s deputy assistant secretary for energy.

Regular marine diesel and jet fuel purchased by the Pentagon around the same time cost just less than $4 a gallon, according to the Defense Logistics Agency’s energy division.

In the Navy’s eyes, this cost is just like the scary-high prices of any brand-new technology, such as the first cell phones or laptop computers.

“As we buy in small, testing quantities, there’s additional cost that most people would understand would logically be there,” Hicks said. “When we go toward bringing these fuels in at operational quantities, we’re only interested in those fuels if they are competitive with petroleum. Period.”

The “green fleet” demonstration is intended to show that an aircraft carrier, its fighter jets and attending warships can operate for two days on alternative fuels.

The carrier Nimitz, which is nuclear powered, and several other ships, including the San Diego-based cruiser Princeton, will participate in the display, which will take place during the Navy’s Rim of the Pacific exercise off Hawaii.

In 2016, the Navy plans to dispatch a similar alt-fueled carrier group on a six-month deployment — the bread and butter of Navy operations.

But if the language in the current defense bill sticks and remains law until 2016, it could torpedo the Navy’s green deployment vision.

Hicks acknowledged that the existing defense bill still allows biofuel purchases for research and testing, but not for deployments — unless the cost difference goes away by then.

It also would block the Navy from making investments in alternative-fuels companies trying to build production plants, Hicks said. Without that military investment, the budding industry will likely grow more slowly, he said.

A February report by Bloomberg New Energy Finance said that some biofuels will be competitively priced with jet fuel by 2018.

People in the business say it might be sooner, even three to five years for some more-advanced formulations of vegetable oil.

One of those is Brian Brokowski, vice president for communications at SG Biofuels, a San Diego company developing jatropha, a subtropical crop that produces oil-bearing seeds.

Already, the company can produce batches of its biofuel crude at $99 a barrel — not too far above petroleum prices. Airlines, including Luftansa and Continental, have flown test flights with jatropha-based fuel.

“In three to five years, with jatropha, that’s the point at which we will begin to see large-scale projects beginning to produce large volumes of plant oil that can be low-cost and competitive with petroleum,” Brokowski said.

The Navy has spent $10 million to $12 million a year for the past few years on its biofuel experiment, Hicks said. An F/A-18 Hornet broke the sound barrier in 2010 while flying on a blend of oil from camelina, a version of mustard seed. The Navy dubbed it the Green Hornet.

In November, a retired destroyer sailed from San Diego to Ventura County on a mix of traditional diesel and algae oil, at the time the largest test of a warship on biofuels.

Navy Secretary Ray Mabus launched his green initiative after being named to the job in 2009 by President Barack Obama.

It’s not just biofuels. Nearly every San Diego Navy base has installed solar panels in recent years. Water-guzzling grass has been replaced by artificial turf.

Camp Pendleton Marines have experimented with solar power at remote operating bases in Afghanistan. One battalion reported in early 2011 that it went from using 20 gallons of fuel daily in its electric generators to less than three gallons. That meant fewer fuel convoys putting Marines in danger.

And if you want to tackle American petroleum dependence, the military is your biggest single target. The United States consumes 22 percent of the world’s petroleum, and the Pentagon is the federal government’s biggest customer for it.

But the United States isn’t importing as much oil as it used to, down from a peak in 2005. In 2011, the U.S. produced just over half of the oil it used, according to the U.S. Energy Information Administration.

About 22 percent of imported petroleum comes from Persian Gulf nations, with Saudi Arabia accounting for most of it. The leading foreign supplier is Canada, which provides 29 percent.

Still, some are calling Congress shortsighted for this cost-cutting move. Stephen Mayfield, director of the San Diego Center for Algae Biotechnology, also called it election-year politicking.

“It’s the Navy that took us from wind power to steam power to petroleum to nuclear. So it’s the U.S. Navy that has led the switch to every new fuel for the last 200 years,” Mayfield said. “And they were clearly the group that was going to do this again.”

The center was established in 2008 as a consortium of researchers from the University of California San Diego and two Scripps institutions to translate laboratory successes into business.

Mayfield said the algae biofuel industry has already created 600 jobs in San Diego County.

The monthlong RIMPAC exercises begin June 29 and San Diego warships will begin departing for it in coming weeks.